Traditionally, in loud noise environments, hearing has been protected by the use of earmuffs or ear plugs. In severe noise environments, the preferred practice has been to use both ear plugs and earmuffs to protect hearing. Workers in noisy environments, despite the danger to their hearing, have often objected to the use of earmuffs and ear plugs because of the sense of isolation that is often felt when such devices are used. Nevertheless, management has usually insisted on the use of earmuffs in noisy industrial environments, because a supervisor can readily check whether a worker in such an environment is using the protection, whereas it is not easy to determine whether a worker is using ear plugs.
This perceived problem with earmuffs and ear plugs was partly responsible for the development of the so-called "electronic earmuff", with which the sound incident upon an ear is controlled using an automatic gain control amplifier. This amplifier receives signals from a microphone and its output signal is used to activate a small loudspeaker mounted adjacent to the ear being protected. The gain of the amplifier is such that sound incident upon the microphone, up to a predetermined level, results in a sound level from the loudspeaker of approximately the same sound level. When the level of incident sound upon the microphone exceeds the predetermined level, the gain of the automatic gain control amplifier acts to suppress the level of sound generated by the loudspeaker to a non-hazardous level. This facility allows speech communication in quiet regions to take place easily but protects the user of the "electronic earmuff" from hazardous noise levels.
Unfortunately, even the electronic earmuff does not provide complete protection of the hearing of a wearer of the earmuff. Sound can "leak" past the earmuff through the natural groove between the jaw and the mastoid bone. Earmuffs can also be disturbed by sudden movements of the head and if they are accidentally knocked. The electronic earmuff has thus been regarded as able to provide only partially adequate protection against loud impulsive sounds--that is, sounds of high intensity which occur for short periods of time, such as the sound when an artillery shell is fired, or a drop forge or a metal press is activated.
One of the known physiological features of the human ear is the automatic contraction of small muscles in the middle ear, noteably the stapedious muscle in response to very loud sounds. When such muscular contraction takes place, the transmission of sound from the ear drum through to the inner ear is attenuated. Since damage to hearing due to noise occurs in the inner ear, anything which reduces the sound energy reaching the inner ear acts as a hearing protector, to a certain degree. The contraction of the muscles in the middle ear attenuates the sound transmitted by about 14 dB at frequencies below about 2,000 Hz. Since the sound energy from artillery firing and the like peaks at about 1,000 Hz, this natural protection is potentially useful in environments subjected to impulsive noise.
Unfortunately, this reflex muscular contraction (a) takes from 100 to 200 milliseconds to become effective, and (b) "adapts" or "fatigues" quickly. Thus protection from the energy from brief intense sounds can only occur if the impulsive sounds are generated in rapid succession, and no protection is provided by this mechanism against the first impulsive sound or from continuous loud sounds.
Although this natural form of hearing protection has been known for many years, its potential for use in noisy environments has been reported only once--in 1962 in US tanks to protect the hearing of the crewmen of a tank when the tank gun was fired.